Make all paths absolute

Convert all file and path names from the command line to absolute paths.
This helps when running tegra-uboot-flasher out-of-tree with a local
bootloader or flash image. This is needed, since tegra-uboot-flasher
cd's to a temporary work directory when executing most commands, which
causes relative path names to no longer be valid.

Signed-off-by: Stephen Warren <swarren@nvidia.com>